We would love to know how to get this working as well.  We have also used solutions such as calls to MSMapi32.ocx (Active-X Control), cmail.exe (a 3rd-party SMTP send-only program), or calls to Outlook (classic version) in the past.  However the "New Outlook" no longer supports this to our understanding.  And it has become more difficult to use a 3rd-party program such as "cmail.exe" with services such as Office365 and Gmail.  And from what I read this has to do with moving to OAUTH2 authentication.  It seems that our clients that have had "cmail.exe" working with 365 for some time continue to work (at least for now), but for those trying to set it up new, they cannot get it working.  And from what Microsoft is announcing, it seems like starting next spring, OAUTH2 is going to be required.  One solution that we have used and seen used with 365 is to setup an SMTP Relay using the information provided in the link below.  However, we would like to move to an integration with MS Graph API but have not investigated that in detail yet, but that seems like maybe a better long-term solution as MS cranks down on security requirements.

How to set up a multifunction device or application to send email using Microsoft 365 or Office 365 | Microsoft Learn

I'm not aware of any samples that work with the O365 REST API. We have the RMNET and JSON samples that are installed with the product. 

Do you have the O365 API setup ready to test some API requests?

Are you able to make any requests to O365 API endpoints from any REST client?

This looks like a good starting point that includes a simple GET request that you could try - https://learn.microsoft.com/en-us/outlook/rest/get-started

Thank you

I send emails a number of ways.
I still use MSMAPI.OCX (with Office 2019 & Windows 11), but you must use the 32 bit version of Word
I also use a royalty free ActiveX sold by Chilkat (Chilkat has a whole cornucopia of useful controls)
It is also possible to use CURL or an app on Linux.